The low-salt transition of chromatin core particles is reversible if the monovalent cation concentration is kept above 0.2 mM. Exposure of the particles to salt concentrations below this value results in a nonreversible secondary transition. The nonreversible changes are relatively slow with a half-time of about 15 minutes. Once exposed to such low ionic strength, the particles then begin to refold with increasing salt in at least two steps over a much higher ionic strength range than is required for the usual low-salt transition. The refolding is very fast, with a half-time less than a minute. Small differences between particles which had or had not been exposed to very low salt persist even when the particles are returned to near physiological ionic strengths.